1. Field of the Invention
The present invention relates to the field of computer systems, in particular, programming language compilers and interpreters of these computer systems. More specifically, the present invention relates to resolving references in compiler generated object code.
2. Background
The implementation of modern programming languages, including object oriented programming languages, are generally grouped into two categories: compiled and interpreted.
In a compiled programming language, a computer program (called a compiler) compiles the source program and generates executable code for a specific computer architecture. References to data in the generated code are resolved prior to execution based on the layout of the data objects that the program deals with, thereby, allowing the executable code to reference data by their locations. For example, consider a program that deals with a point data object containing two variables x and y, representing the x and y coordinates of a point, and further assume that the variables x and y are assigned slots 1 and 2 respectively, in each instance of the point data object. Thus, an instruction that accesses or fetches y, such as the Load instruction 14 illustrated in FIG. 1, is resolved to reference the variable y by the assigned slot 2 before the instruction sequence is executed. Particular examples of programming language compilers that generate code and resolve data references in the manner described above include C and C++ compilers.
This "compiled" approach presents problems when a program is constructed in pieces, which happens frequently under object oriented programming. For example, a program may be constructed from a library and a main program. If a change is made to the library, such that the layout of one of the data objects it implements is changed, then clients of that library, like the main program, need to be recompiled. Continuing the preceding example, if the point data object had a new field added at the beginning called name, which contains the name of the point, then the variables x and y could be reassigned to slots 2 and 3. Existing programs compiled assuming that the variables x and y and are in slots 1 and 2 will have to be recompiled for them to execute correctly.
In an interpreted language, a computer program (called a translator) translates the source statements of a program into some intermediate form, typically independent of any computer instruction set. References to data in the intermediate form are not fully resolved before execution based on the layout of the data objects that the program deals with. Instead, references to data are made on a symbolic basis. Thus, an instruction that accesses or fetches y, such as the Load instruction 14' illustrated in FIG. 1, references the variable y by the symbolic name "y". The program in intermediate form is executed by another program (called an interpreter) which scans through the code in intermediate form, and performs the indicated actions. Each of the symbolic references is resolved during execution each time the instruction comprising the symbolic reference is interpreted. A particular example of a programming language interpreter that translates source code into intermediate form code and references data in the manner described above is the BASIC interpreter.
The "interpreted" approach avoids the problems encountered with the "compiled" approach, when a program is constructed in pieces. However, because of the extra level of interpretation at execution time, each time an instruction comprising a symbolic reference is interpreted, execution is slowed significantly.
Thus, it is desirable if programming languages can be implemented in a manner that provides the execution performance of the "compiled" approach, and at the same time, the flexibility of the "interpreted" approach for altering data objects, without requiring the compiled programs to be recompiled. As will be disclosed, the present invention provides a method and apparatus for resolving data references in compiler generated object code that achieves the desired results.